Ronald Koeman. (AFP/File/Olaf Kraak)
Koeman resigned as Feyenoord boss last month and has agreed a three-year contract with the Premier League club to replace Mauricio Pochettino following the Argentine's move to Tottenham.
The 51-year-old admitted he decided to join Southampton after a positive meeting with Saints executive director Les Reed convinced him his football philosophy can thrive on the south coast.
"I am thrilled to be the manager of Southampton Football Club," Koeman said.
"The opportunity to continue the development of this famous football club was simply too good to turn down.
"From my very first meeting with Les Reed, I could see clearly that this was the right club for me.
"We share the same philosophy of football and the same belief in how to develop the club."
Koeman will be joined at St Mary's by his older brother Erwin, who has been named assistant manager, and Jan Kluitenberg, who will be an assistant and a fitness coach.
During Koeman's three-year spell at Feyenoord he led the team to consecutive top-three finishes in the Dutch league and Reed believes he is the right man to continue the impressive progress Southampton had made under Pochettino.
"From the moment we met with Ronald, it was clear he had the vision and ambition to take our club forward, that he shared our football philosophy and could see the opportunity to move our club to the next level," Reed said.
"The club, the players and the fans will all be excited to have a man of Ronald's experience at the very highest level joining our club.
"Ronald's passion for the opportunity here at St Mary's was very clear, and we look forward to continuing our growth with him as part of our team."
One of Koeman's first tasks will be persuading several of the club's key players to ignore offers from bigger teams.
England striker Rickie Lambert joined Liverpool recently, while defenders Luke Shaw and Dejan Lovren are being chased by Manchester United and Liverpool respectively.
Saints have also turned down two approaches from Liverpool for England midfielder Adam Lallana.
Koeman will discover his first opponents in English football on Wednesday when the 2014-15 Premier League fixtures are released.
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